In a UMTS (Universal Mobile Telecommunications System) network, attempts are made to optimize features of the system, which are based on W-CDMA (Wideband Code Division Multiple Access), by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), for the purposes of improving spectral efficiency and improving the data rates. With this UMTS network, long-term evolution (LTE) is under study for the purposes of further increasing high-speed data rates, providing low delay, and so on (non-patent literature 1).
In the third-generation system, a transmission rate of maximum approximately 2 Mbps can be achieved on the downlink by using a fixed band of approximately 5 MHz. Meanwhile, in an LTE system, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. Furthermore, with the UMTS network, successor systems of LTE are also under study for the purpose of achieving further broadbandization and higher speed (for example, LTE advanced (LTE-A)).
Also, in a system of an LTE scheme, a user terminal reports the channel state to allow a radio base station to perform optimal scheduling. As a parameter for reporting this channel state, downlink quality information (CQI: Channel Quality Indicator) for use in adaptive modulation/demodulation and coding process (AMC: Adaptive Modulation and Coding scheme) is defined. Scheduling is executed by feeding back channel state information (CSI) including this CQI from the user terminal to the radio base station.